The present invention relates to a variety of electronic instruments, such as electronic watches, portable communication instruments, or electric home appliances, and particularly to preventing an erroneous operation from occurring when the voltage of a power source circuit of a low power consumption instrument has been lowered.
Hitherto, in electronic watches, portable communication instruments, and other various electronic instruments, as signals come to be digitally processed, a frequency-dividing circuit, a central processing unit (CPU), a flip-flop circuit, or the like has been used.
FIG. 9 is a block diagram of a conventional electronic instrument. In FIG. 9, a power source circuit 901 is made up of a thermoelectric conversion element 902 as an electric power generating means, and a rectifying circuit 903 for preventing countercurrent. Electric power generated by the thermoelectric conversion element 902 is stored in a storage means 904 through the rectifying circuit 903. A load circuit 905 made up of a MOS transistor, a bipolar transistor, and the like operates using the generated electric power or stored electric power as a power source. For example, in the case where the electronic instrument is a clock system of an electric watch, this load circuit is made up of an oscillating circuit, a frequency-dividing circuit for frequency dividing an output signal of the oscillating circuit, a time display means for displaying a time on the basis of an output signal of the frequency-dividing circuit, and the like.
In such a conventional electronic instrument, for the purpose of making the electronic instrument miniaturized and lightweight, the power source circuit 901 is also required to be made miniaturized and lightweight, and with this, there is a tendency that the power supply capability of the power source circuit 901 is decreased. On the other hand, by virtue of need of prolonging the operation time of the electronic instrument, the storage means 904 is forced to have its capacity made large. These are noticeable especially in a portable electronic instrument such as an electronic watch or a portable telephone.
In such a case, when the storage means 904 is charged, from a non-charged state, with electric power supplied from the power source circuit 901, the power source voltage to the load circuit 905 is not quickly raised, but a long time is required until the power source voltage is increased up to a voltage at which the load circuit 905 operates normally. Thus, a period of time when the power source voltage remains in the vicinity of an operation start voltage at which the operation of the load circuit 905 becomes unstable, becomes long. Accordingly, there have been problems in that not only does the start of operation of the load circuit 905 become delayed, but also the load circuit 905 can perform an erroneous operation.
In order to prevent this erroneous operation, there is generally used a method in which a power on clear circuit is used, which detects the quick increase of voltage at the start of power supply and outputs a clear signal, and with the start of power supply, the clear signal is supplied to a frequency-dividing circuit, a CPU, and the like to reset them into a predetermined initial state, and then, a normal operation is started.
However, even if the power on clear circuit is used, the power source voltage is not quickly raised because of the foregoing reason, so that a sufficient clear signal can not be generated and it has been difficult to solve the erroneous operation of the load circuit 905.
Besides, in the case where the load circuit is a clock system, a MOS transistor having large driving capability is used in the load circuit to the power source in order to drive a stepping motor. Thus, when the power source voltage of the load circuit enters into a voltage range where the operation of the circuit is unstable, a large current flows by a through current. In this case, the electric power supplied from the power source circuit 901 is consumed by the above through current, so that there is a case where the storage means 904 can not be charged. When the through current flows once, the power source voltage does not become larger than a voltage at which the through current of the stepping motor driving MOS transistor balances with the power supply capability of the power source circuit 901. Thus, there has been a problem in that if the balancing voltage is less than an operation voltage of the load circuit 905, the load circuit 905 does not start a normal operation for a long time.
Besides, some load circuits 905 have such a mechanism that when a power source voltage is low, more current flowns. In this case, when the power source circuit 901 starts to supply electric power, the voltage of the storage means 904 is gradually increased, so that more current flowns when the voltage is low. Thus, there has been a problem in that voltage drop occurs and the voltage is not increased.
Further, even in an electronic instrument which does not include an electric power generating means and a large capacity storage means as described above, in such a case that a relatively large smoothing capacitor is used to stabilize the power source voltage, quick rising of the power can not be obtained. Thus, there has been a problem that a sufficient clear signal can not be obtained by the foregoing power on clear circuit, so that an erroneous operation occurs.